Nailing those Galaxies

Thank's to the help I've received here I've created rules to turn the polar coordinates into cartesian coordinates but, say I had the coordinates for the galactic centre for Andromeda and the Milky Way and I also had coordinates for certain stars. I want to put a limit on my database that not allow me to accidently place a star that should be in the Milky Way into the Andromeda galaxy.

Well, the database you linked actually describes the boundaries between constellations -- the imaginary lines drawn on the sky to separate what astronomers, by convention, call the constellations. These boundaries have no physical meaning and have nothing to do with a star's parent galaxy. Perhaps you are mistaking the constellation Andromeda for the Andromeda Galaxy, a large galaxy which just happens to be in that constellation.

The bottom line is that all the stars you see in the night sky are in our own galaxy. You'd need a pretty good telescope to pick out individual stars in the Andromeda galaxy.

Seeing as I have no idea where you've gotten your datasets, or what they actually mean, I can probably do little to help you. You may want to begin at the beginning, and tell us what you've done and what you're trying to do.

A friend and I had an idea for an RPG (Role-playing game) and it grew and grew. It's been 6 years and it's in it's 4th edition.

Now, I love stats and Math, he doesn't. So when I proposed mapping out the stars I was given a blank look.

As with everything I started out with researching and I found lots of data with stella coordinates in the Polar format. I figured if I could convert them into cartesian coordinates I could calculate the distance between the stars for the purpose of travel time within the game. I've done that so that's no longer a problem.

Within my database I have different Galaxies (with the coordinates of their centres) as well as the type of galaxy they are in one table.

In another I have star coordinates(and there type).

I could just make them up but I'd rather have the whole mechanics based upon as much factual data as is feasible. Now, when I place a star into my database I want it to automatically select it's parent galaxy. To do that I need to know the boundary limits, in cartesian coordinates, of the galaxies I place into my database. Hence my previous post.

I love most things to do with science but I'm not an expert with any stretch of the imagination. And so, when I need an educated answer, I go to the only experts I know; which is the people here.

This project is going to take a long time, I know this, that's why I'm trying to link everything together as efficiently as possible at the beginning to minimise the errors in the future.

Hope you can help

Oh yes...

Perhaps you are mistaking the constellation Andromeda for the Andromeda Galaxy, a large galaxy which just happens to be in that constellation.

I could just make them up but I'd rather have the whole mechanics based upon as much factual data as is feasible. Now, when I place a star into my database I want it to automatically select it's parent galaxy. To do that I need to know the boundary limits, in cartesian coordinates, of the galaxies I place into my database. Hence my previous post.

Chroot already answered this, or else you are using the words "galaxy" and "Star" incorrectly. Your small quote I used above is about placing a star in your database. Chroot already explained that every star you can see is in just one galaxy, our own! So, the "parent galaxy" for all of your stars is our own. Otherwise, I don't understand if you mean galaxy or consellation?

Stars in the Milky Way galaxy,
stars in the Andromeda galaxy and
stars in galaxy X

I want to have fixed coordinates for individual galaxies (their centres) AND fixed coordinates for individual stars.

IF I can define the volume of space that holds a particular galaxy (with cartesian coordinates)then I should be able to state that if a star is within that volume of space then it is part of that galaxy

Stars in the Milky Way galaxy,
stars in the Andromeda galaxy and
stars in galaxy X

I want to have fixed coordinates for individual galaxies (their centres) AND fixed coordinates for individual stars.

IF I can define the volume of space that holds a particular galaxy (with cartesian coordinates)then I should be able to state that if a star is within that volume of space then it is part of that galaxy

That won't work, because too many galaxies have "foreground" stars, and sometimes even "background" stars, quasars and other background galaxies, galaxy clusters, etc. But, even in that volume, three dimensions, individual stars are usually impossible to resolve.

Originally posted by Daminc As with everything I started out with researching and I found lots of data with stella coordinates in the Polar format.

Okay, let's begin here. What stars are listed? I'm going to make the (educated) assumption that they are the coordinates, probably in the form (right ascension, declination, distance), of stars in the Milky Way galaxy. No one, to my knowledge, has made a catalog of stars in other galaxies. Why not?

Well, you first must understand something about measuring distance. Measuring distance is one of the most difficult things astronomers have to do. Think about why: a nearby dim star and a distant bright star look the same to us. Since stars come in all kinds of inherent brightnesses (called luminosities), you can't use brightness as an indicator of distance.

Within some fairly small distance from the Earth, we can use an effect called parallax to measure distances. When you view the same object from two or more different positions, the object appears in a different position relative to distant background objects. You have two eyes -- each eye sees a slightly different picture of the things in front of you. Your brain uses those slightly different pictures to measure distances to things. Astronomers do the same thing. They precisely measure a nearby star's position relative to much more distant stars. Then they wait six months (so the Earth is on the opposite side of the Sun) and they do it again. The small change in the star's position relative to the background allows them to calculate distance.

All of the naked-eye stars, and quite many others, have had their distances measured this way. The most recent experiment conducting precision parallax studies was the Hipparcos satellite, which I believe measured about 10,000 distances. Chances are, you have a catalog of these nearby stars.

Now, things get substantially harder when you start dealing with stars in other galaxies. Parallax no longer works -- the angles are far, far too small to be measured accurately (with existing technology anyway). Astronomers generally measure the distances to the galaxies using tools like Cepheid variable stars, which are a special class of stars which vary in brightness in such a way that we can determine their luminosities from the period of their variation.

Galaxies are so far away (millions or billions of light-years) that astronomers generally just give one distance estimate for the entire galaxy, and do not generally ever bother with the distances to individual stars.

I suspect that you have a catalog of nearby galaxies (was that you asking about the Tully catalog a while ago?), which gives the 3D positions of some galaxies.

What you're failing to realize is that your datasets do not overlap. Your star catalog lists stars near the Earth (some hundreds of light-years away), while your galaxy catalog lists entire galaxies (some millions of light-years away). There are no stars in your star catalog that are not in the Milky Way.

I do find this a bit confusing..

Good of you to mention this. Let's talk about constellations a bit.

The sky appears two-dimensional to us -- we can imagine a giant sphere surrounding the earth, with white dots painted on its inside surface. We can ascribe two coordinates to any point on the inside surface of this imaginary sphere. In a very similar way, we can ascribe two coordinates (latitude and longitude) to any point on the surface of the earth.

Now for the sake of naming, astronomers have chosen to break the sky up into small interlocking regions -- like a jigsaw puzzle. In a similar fashion, we've also chosen to break up the surface of the Earth into interlocking regions. We call the regions on the Earth countries, states, counties, and so on. These so-called 'political boundaries' are not physical -- you don't trip over a big black line when you cross from one state to another. The regions on the sky are called 'constellations,' and the boundaries are no more physical than the boundaries between states.

The states have cities within them; you would then say that San Francisco is within the state of California. Constellations have stars and galaxies and so on with them, too -- so you might say that the Andromeda Galaxy is within the constellation of Andromeda.

The only difference is distance. All of the cities on the Earth are, quite obviously, on the surface, at more or less the same altitude -- the same distance, say, from the Earth's center.

The stars and galaxies and other objects inside a constellation, however, can be a vastly different distances. The stars in a constellation are pretty close to us. The galaxies are very, very much further -- and the quasars further still. The things in a constellation are not all physically related.

So it sounds to me like you have a mixture of concepts and datasets which isn't going to accomplish much. I believe you have:

1) a catalog of nearby stars' 3D positions (all of which are in the Milky Way).
2) a catalog of nearby galaxies' 3D positions (none of which contain any specific star information).
3) a list of the boundaries between the constellations on the sky.

(Keep in mind that 'nearby' means very different things for stars and galaxies! A nearby star is a few hundred light-years or less distant, and a nearby galaxy is a few tens of millions of light-years or less.)

With this information, you're not going to be able to accomplish what you want -- what you're looking for is data on the positions of individual stars in other galaxies (like Andromeda), and this data simply doesn't exist. It isn't very important to us astronomers!

Can I find the coordinate limits on just the Milky Way Galaxy for example, I've read on estimate that says that it is about 100,000 light years in diameter and about 1000 (3000? at the centre?) light years thick.

...the halo which is a spherical region, centred on the nucleus, with a radius of about 50000 light years. This halo contains very old stars, produced early on when the galaxy was still forming. Most of these stars are in vast collections called globular clusters

This bit isn't essential.

If I can do that with the Milky Way then later on I can use a bit of poetic licence to determine other galaxies

Looking back on what I've wrote I decided to get the catesian coordinates for the centre of the Milky Way and came up with this:

Originally posted by Daminc
Can I find the coordinate limits on just the Milky Way Galaxy for example, I've read on estimate that says that it is about 100,000 light years in diameter and about 1000 (3000? at the centre?) light years thick.

The Milky Way is about 100,000 ly in diameter, but I think the 3000 ly figure refers to the thickness at our location (28,000 ly from the center of the galaxy). The thickness of the galaxy is somewhat hard to pin down as it depends on what you use to define the upper/lower edges (stars? molecular clouds? dark matter?)

What do these numbers mean? To make sense of coordinates, you need to specify both the origin and the orientation of your x, y, and z axes.

The Cartesian coordinates are converted from Polar coordinates where the zero points defined by the point where the Sun crosses the equator on the first day of Spring for RA and the celestial equator and poles for declination. RA is measured in hours, minutes, and seconds (24 hours equals 360 degrees of "longitude"; RA increases in the eastward direction). Declination is measured in degrees (0 at the equator; 90 at the Celestial North Pole; objects below the celestial equator are assigned negative declinations).

... and unit value

The scale is in light years.

The thickness of the galaxy is somewhat hard to pin down as it depends on what you use to define the upper/lower edges (stars? molecular clouds? dark matter?)

I'd use the stars to define the edges and from how I understood what I read about the thickness, the 1000ly was refering to where we are and the 3000ly was the centre.

Okay, stop calling them "polar coordinates." They are spherical coordinates. And astronomers call them "equatorial" coordinates, not polar or spherical, to distinguish them from many other coordinate systems in common use, like galactic and so on.

In any event, you didn't answer the question in entirety. I gather that the Earth is the origin of your coordinate system, and the unit length is light years. Where do your x-, y-, and z-axis unit vectors point?

Okay, stop calling them "polar coordinates." They are spherical coordinates.

In math I think we have spherical polar coordinates and cylindrical polar coordinates and it was my understanding that 'spherical polar coordinates' and 'spherical coordinates' are the same except I took away the 'spherical' because of the context.

I think that X would follow in the direction RA=0 and Dec=0 with the Y and Z 90 degrees out of phase

I think that X would follow in the direction RA=0 and Dec=0 with the Y and Z 90 degrees out of phase

Okay, you're a third of the way there. You've given us one of the unit vectors. The other two vectors, however, could be "90 degrees out of phase" in an infinite number of ways. It would be instructive for you to give us the other two vectors, too.

I'm not being hard on you -- I just think you could stand to have some of your ideas tightened up a bit. The first step is often to begin using the right terminology and methodology.